The backing tracks are tuned to A4 = 440 Hz. You can set it anywhere between 415 and 466 Hz to match an instrument at a different pitch.

 

On Mac, open Preferences > General. On Windows, open Settings from the cog at the top, or File > Settings, and find it under General.

 

To tune an instrument to iReal Pro, play a long A5 chord at a slow tempo, or set the count-in Click 1 to Keynote so it sounds the root of the key. See Play a song.

 

For more, see Sound output and audio devices on the iReal Pro website.